The field of the invention relates generally to turbomachines and, more particularly, to turbomachine seal systems that include abrasive materials applied to a tip of an airfoil and high temperature resistant and erosion resistant materials applied to a static portion and methods of applying the same.
At least some known turbomachines are turbine engines that include at least one stationary assembly extending over at least one rotor assembly. The rotor assembly includes at least one row of circumferentially-spaced, rotatable, metallic turbine blades or buckets. Also, at least some known turbine engines are gas turbine engines that also include at least one row of circumferentially-spaced, rotatable, metallic compressor blades. The buckets and blades include metallic airfoils that extend radially outward from a platform to a metallic tip. Many of such metallic airfoils are fabricated from materials such as nickel (Ni) and cobalt (Co) alloys.
Some known stationary assemblies of turbine engines include surfaces that form metallic shrouds that may be routinely exposed to a hot gas flux. Some of such metallic surfaces include an applied metallic-based MCrAlY coating and/or an applied ceramic thermal barrier coating (TBC) that forms a shroud over the stationary assembly. Alternatively, some such metallic surfaces include applied ceramic matrix composites (CMC) with, or without, a protective thermal barrier coating.
The metallic tips and the metallic shrouds define a tip clearance therebetween. However, such turbine engines that include both metallic shrouds and metallic buckets are configured with tip clearances that are sufficiently large enough to facilitate rub-free engine operation through the range of available engine operating conditions. However, such tip clearances are only suitable for low-temperature and low-efficiency turbine engines and would not be suitable for higher-temperature units that need higher efficiencies.
Some of the other known turbine engines include abradable shrouds formed over the stationary assembly. Typically, such shrouds are formed with a patterned abradable thermal barrier coating (TBC), including a dense vertical cracking (DVC) form of TBC. The tips are not coated and they abrade the shrouds as the rotor assembly rotates within the stationary assembly because the hardness value of the tips is greater than the hardness value of the shroud coating. Subsequently, the abradable shrouds and the tips define a tip clearance therebetween. The relatively less hard abradable TBC shroud coatings decrease a potential for damage to the relatively harder bucket/blade. However, because of the relative softness, such shroud coatings are prone to material loss to due to particulate erosion, especially along the patterned-contours. To counter particulate erosion, some turbine engines include more sturdy erosion resistant DVC-TBCs that are abraded by the metallic buckets/blades. However, due to the increased hardness, such DVC-TBC coated shrouds facilitate increased bucket/blade tip wear during rubbing, and have shown the tendency to spall due to rub-induced temperature rises.
Some of the remaining known turbine engines include similar abradable TBC shroud coatings formed over the stationary assembly and the bucket/blade tips include an abrasive material formed thereon that has a greater hardness value than the bucket/blade material and the abradable coating. The abrasive material abrades the shroud coatings as the rotor assembly rotates within the stationary assembly. The abradable shroud coatings and the abrasive tips define a tip clearance therebetween. The tip clearance is small enough to facilitate reducing axial flow through the turbine engine that bypasses the blades and buckets, thereby facilitating increased efficiency and performance of the turbine engine. The tip clearance is also large enough to facilitate rub-free engine operation through the range of available engine operating conditions. Moreover, as described above, many such TBC materials are erosion-resistant DVC-TBCs and have shown the tendency to spall due to rub-induced temperature rises.
In addition, some of those known turbine engines that include abradable TBC shrouds and an abrasive material formed on the buckets/blades use either silicon carbide (SiC) or cubic boron nitride (cBN) for the abrasive material due to their hardness characteristics. However, for temperatures above approximately 927 degrees Celsius (° C.) (1700 degrees Fahrenheit (° F.)), cBN becomes unstable and is prone to oxidation. Also, while SiC is better suited to survive temperatures in excess of approximately 927° C. (1700° F.), SiC abrasives include free silicon that may attack the Ni/Co alloy substrates.